Just a couple of comments from my experiences with my 6x12 Fastrack layout...
- Being able to move between loops will add a great deal to your options.
- Likewise being able to reverse direction from either direction of travel adds a lot to your operations.
- I deeply regret not planning an O72 mainline.
I'll write more later but no O72 mainline on the layout means my PM1225 is running on a 7x11 floor oval. Better than nothing but not fun. Since I will need to severely redo my benchwork anyway to accommodate O72, I am also looking into what I'd have to do for O84 and O96.